Nyamsus is the family cat of the Kousaka household and a quiet but important presence in the daily life of the family, especially for Junpei Kousaka. She is a plump domestic cat who presents herself as ill-tempered and, at the start of the story, appears to want nothing more than to make life miserable for Junpei, who is allergic to cats and dislikes them. This harsh impression, however, is not the whole truth. As the series progresses, her behavior is shown to be a cover for genuine affection, and she is secretly deeply devoted to and caring toward her master.

Her background ties her to another character in an unexpected way. Nyamsus was originally a kitten left in a comfortable box by Kaede Mizuno, who had found her but could not keep a cat in her own home because of the many dogs living there. A sign attached to the box asked for someone to give the kitten a home, and the Kousaka family ultimately took her in. This origin story gives Nyamsus a meaningful connection to Kaede and adds emotional depth to her position within the household.

In the story, Nyamsus serves as Junpei's guide after he is cursed with the ability to understand cats. She is the one who regularly informs him of the troubles faced by cats in the neighborhood and directs his attention to the requests he must fulfill in order to lift the curse. She is usually accompanied by Tama, another cat from the local temple, and the two of them work together to bring matters to Junpei's attention. The first request Junpei receives comes from both Nyamsus and Tama, who ask him to help the stray cats in the local park that have been tormented by a stranger. In this way, she functions as a kind of liaison between Junpei and the wider feline community, steering him toward the good deeds that form the central task of the series.

Her motivations are rooted in care rather than malice. Though she often acts gruff and dismissive toward Junpei, she genuinely wants him to succeed and looks out for his wellbeing, even as she maintains her sharp-tongued manner. She also holds a respected place among the local cats, as most of the felines in the neighborhood think highly of her, which allows her to gather information and act as an intermediary on their behalf.

Her most important relationship is with Junpei. Their dynamic is defined by her outward hostility and his allergy-fueled aversion to cats, but beneath that friction lies a strong bond that becomes more visible as events unfold. Her companionship with Tama is also significant, as the two cats are frequently seen together and cooperate in bringing requests to Junpei. Her connection to Kaede is chiefly revealed through her backstory, since Kaede was the one who left her for adoption, a detail that links the two characters even though they do not interact as closely.

Nyamsus does not develop dramatically in terms of personality, but her character is gradually reframed as her true feelings are revealed. The initial impression of her as a spiteful or troublesome pet gives way to a more sympathetic understanding of her as a devoted, protective presence who cares deeply for the people around her. Her abilities are not supernatural in nature; she relies on her knowledge of the neighborhood, her standing among other cats, and her ability to communicate with Junpei to play her part in guiding him through the tasks he must complete.